Which instruction should you give to an aircraft if their transponder appears inoperative?

When instructing an aircraft that their transponder appears inoperative, it's essential to communicate effectively to ensure safety and compliance with air traffic control regulations. The correct guidance involves informing the pilot of the perceived issue and providing them with the next steps to resolve or circumvent the problem.

By stating that the transponder seems to be inoperative, you communicate clearly the situation at hand, prompting the pilots to take action. Advising them to reset the transponder is a constructive step because it can often resolve temporary glitches. Recommending that they "squawk" a specific code—usually a transponder code assigned by air traffic control—ensures that they can continue to communicate their presence and intentions to ATC, enhancing situational awareness.

The other options do not effectively address the problem or provide appropriate guidance. Simply stating that their transponder appears fine could mislead them and might overlook a real issue. Suggesting a frequency change does not inherently rectify the transponder situation and could introduce additional confusion. Telling them to squawk 1200, which is a VFR code, is not suitable unless they are operating under VFR conditions, as it may not provide the necessary tracking and situational awareness that ATC requires in controlled airspace.